EFFECTS OF PESTICIDE, SOIL, AND RAINFALL CHARACTERISTICS ON POTENTIALPESTICIDE LOSS BY PERCOLATION - A GLEAMS SIMULATION

Potential pesticide loss in soil percolate is influenced by pesticide persistence and sorption by soil constituents (organic matter). Pestic ide persistence, expressed as half-life (t1/2), changes with soil dept h as microbial activity and soil properties change. Little is known, h owever, how these changes influence potential pesticide transport out of the root zone. Objectives of this study were to investigate relativ e differences in potential pesticide losses from the root zone by perc olation due to 1) different soil surface and subsurface textures and p esticide t1/2, and 2) interactions between pesticide t1/2 and timing o f rainfall after pesticide application. The GLEAMS (Groundwater Loadin g Effects of Agricultural Management Systems) model and a 50-year hist orical rainfall record at Tifton, Georgia, were used to simulate pesti cide losses by percolation from three soils ranging in surface texture from sand to sandy clay loam. Hypothetical pesticides had surface t1/ 2 of 5, 15, 30, and 60 d and a range of subsurface t1/2 (2.5-360 d), a nd were applied to continuous corn (Zea maize, L.) at 2 kg ha-1 as sur face spray at planting each year on 1 April. Simulated pesticide losse s by percolation increased with increased surface and subsurface t1/2, and decreased with increased K(oc) (adsorption constant based on soil organic matter) values. Potential pesticide leaching was greatest for Lakeland sand and least for Greenville sandy clay loam. Rainfall timi ng affected simulated pesticide loss by percolation, especially for no npersistent pesticides. For short pesticide t1/2 (0-5 d), excessive ra infall events within 1 t1/2 were largely responsible for simulated pes ticide loss by percolation. Results indicate that changes in pesticide t1/2 in surface and subsurface horizons of different soils influence potential pesticide leaching from the root zone, and models (i.e., GLE AMS) can be used to provide comparative analysis of soil-pesticide-cli mate interactions. For example, depending on soil type and pesticide K (oc) and surface t1/2 values, potential leaching losses increased two to seven times as subsurface t1/2 increased six times.